Approach to the vomiting child practice

A 7-month-old girl has had six hours of episodes in which she suddenly screams, draws her knees up and stiffens for a minute or two, then lies pale and floppy until the next episode. She has vomited twice; the vomit is milky. She has passed one small stool containing mucus and dark blood. She is afebrile. The abdomen is soft with a vague fullness in the right upper quadrant, and there is no organomegaly and no hernia. Which one of the following investigations is most likely to establish the diagnosis?